The present invention relates to a sound reducing construction for shielding sound generated upon operation of a machine such as a printer, and more particularly, to a type thereof adapted to be provided at a box which accommodates therein a printer for confining sound in the box.
As shown in FIG. 1, a printing machine such as a line printer P is accommodated in a box 100 consisting of a main body 102 and a cover 101 those being formed of metal plates. Within the main body 102, the printer P is accommodated, and the cover 101 is pivotally connected to the main body 102 at its rear portion 101R so as to cover a top surface portion of the printer P. During operation of the printer P, the rectangular lower open end face 101a of the cover 101 is brought into contact with a rectangular open end face 102a of the main body 102 by the pivotal motion of the cover 101, to thereby provide a confined space defined by the main body and the cover.
In order to avoid sound leakage from the box interior to outside, a sound reducing construction has been provided at an interface between the main body 102 and the cover 101. For this, one conventional sound reducing construction is shown in FIG. 2. FIG. 2 shows closed state of the cover 101 and the man body 102 as viewed in a direction indicated by an arrow A in FIG. 1, and left and right side margins of FIG. 2 designate box internal and external portions, respectively. According to the conventional sound reducing construction, a packing unit 103 is provided between the cover 101 and the main body 102. To be more specific, the packing unit 103 includes a resilient packing member 103a, a bolt 103b, packing holder plates 103c and a magnetic member 103d. Inner surfaces of the cover 101 and the main body 102 are provided with sound absorbing members 106, and the side end portion of the metal cover 101 is bent by a right angle to provide a packing attaching portion 101A. The packing member 103a formed of soft vinyl chloride has one end portion 103a' fixedly attached, through the packing holder plates 103c, to the packing attaching portion 101A by the bolt 103b. Another end portion 103a" of the packing member 103a is connected to the magnetic member 103d such as a rubber or plastic magnet which is in contact with a bent plate 102A of the main body 102.
Accordingly, the packing unit 103 seals the interface between the cover 101 and the main body 102 when the cover 101 is closed.
In the conventional sound reducing construction, attention should be drawn to "mass law of sound insulation". That is, the rule states that sound insulation for a single wall is determined almost wholly by its weight per unit area, and doubling the weight of the partition increases the insulation by 5 decibels. In this connection, the packing unit 103 provides a sound transmission loss lower than that of the metal plate portion, so that sufficient sound insulation or reducing effect may not be obtainable at the packing portion. Further, the thickness portion "t" of the packing member 103a merely serves as the partition defined in the law, and therefore, sound leakage may occur through the packing portion. Furthermore, according to the conventional structure, it would be rather difficult to conduct clamping work for securing the packing member 103a to the cover 101 by using the bolt 103b, since the packing member 103a must be held with being interposed between the packing holder 103c and the packing attaching portion 101A when the bolt 103b is rotationally advanced.